1. Field of the Invention
The present invention relates to a driving control apparatus which controls a brushless motor.
2. Description of the Related Art
Causes of vibration and noise of a brushless motor include torque variation. In particular, when performing a driving of 120-degree energization method, a vibration or noise is generated due to torque lowering at the time of commutation.
In order to reduce the torque lowering at the time of commutation, there has been suggested a technique in which a predetermined overlap time period is set at the time of phase switching before and after the commutation (refer to JP 2008-118830 A).
In JP 2008-118830A, when turning on/off a switching element of an inverter apparatus, an upper arm signal and a lower arm signal are made to be PWM (Pulse Width Modulation) signals in the overlap time period, so that an inclination of current change, which is generated at the time of the phase switching, is made to be gentle, thereby suppressing the vibration and noise.
However, the technique of JP 2008-118830 A has following problems.
That is, the PWM signal in the overlap time period is generated based on a carrier frequency. Therefore, it is difficult to arbitrarily adjust a pulse width of the PWM signal. Although the inclination of the current change is made to be gentle, only the stepwise adjustment is possible, so that the suppression effect of the vibration and noise is restrictive.
Also, the PWM signals are generated both at the upper arm side and the lower arm side. Thereby, an operation of a driving circuit of generating the PWM signal is complicated, so that a circuit size may be increased.
Further, the power supply current may be sharply decreased in the overlap time period, so that the torque variation may occur.